The present invention relates to in an improved counter current extractor incorporating a screw conveyor, and to an improved process for the counter current, extraction of soluble or dispersable materials from a substrate using such an extractor.
Counter current extractors are well known in the food processing industry for the continuous extraction of liquids, solubles, and fine particlulate matter from associated solids. Such diffusers generally comprise a pair of parallel counter-rotating screw conveyors mounted within an inclined or vertical elongate housing, which takes the form of a trough or totally encased tube. Material to be processed, such as chopped sugar beet, is fed into the lower end of the housing and carried upwards by screw rotation while an extracting liquid (usually water) is fed into the top of the housing and flows downwards under gravity. Single screw counter current extractors are also known, but are recognized as being relatively inefficient because the solid matter tends to build up on one side of the housing, while the liquor tends to flow relatively unimpeded down the other side. A similar problem may be experienced with double screws, but to a lesser extent, because the counter rotation encourages more even distribution of solids over the width of the screws.
Single screw conveyors offer the important advantages of simplicity and cheapness as compared with twin screw counter current extractors, and attempts have been made to improve the efficiency of such single screw extractors.